THIS invention relates to the manufacture of plastics material vessels, particularly vessels for containing or conveying corrosive fluids. Such vessels may be used, for example, as reaction vessels in part of a chemical process facility, for pipes, or in many other applications.
Fibre reinforced plastics (FRP's), in various forms are used extensively as structural materials in a wide range of applications in, amongst others, the aerospace, transport, electrical, mining and chemical industries. These materials have, in certain applications, some advantages over traditional metallic materials including improved specific mechanical properties, manufacturability and corrosion resistance. This has led to widespread use of such materials in structures (eg containers and piping) used for the containment and reticulation of certain chemically corrosive mixtures. While in the majority of applications the materials have performed successfully, there have been instances of premature failure in service. In many cases these failures have been attributed to the mechanisms of environmentally assisted cracking including direct corrosion and erosion mechanisms. For a given chemical environment the rate of crack growth associated with this phenomenon has been shown to increase exponentially with the tensile stress level in the material exposed to the environment. Thus, failures by environmentally assisted cracking are much more likely to occur in cases where the exposed surface of the material in contact with corrosive agents is subject to tensile stress.
Tensile stresses on the exposed surfaces of structures may arise from applied mechanical loading or constrained thermal expansion. Over and above these, however, residual tensile stresses may exist in the exposed material. These stresses may have a number of sources but it has been shown that a major source of such stresses is constrained post-cure shrinkage of the material after or while being subjected to elevated temperatures. Such constraint can arise out of partial post-curing of the material due to temperature gradients developed in service or by differential shrinkage of layers within the material. Stresses of this nature, in excess of 20 MPa, have been measured in laboratory tests, and field tests have shown that similar stresses occur in practical situations. Such levels of stress have been shown to be sufficient to cause rapid failure by environmentally assisted cracking in certain chemical environments.
Such failures, which usually require premature removal of the structure from service, have been known to occur within months of structures going into service, whereas the design life of the structures might typically be more than five years. This obviously has extensive financial implications due to both replacement costs and production losses. Thus, a means of improving the situation would allow a significant saving in costs and increases in production for users of structures subject to environmentally assisted cracking.